Background: Divergent risk factors exist for premenopausal and postmenopausal breast cancers, but it is unclear whether differences by age exist among postmenopausal women.
Methods: We examined relationships among 190,872 postmenopausal women, ages 50-71 years recruited during 1995-1996 for the NIH-AARP Diet and Health Study, in whom 7,384 incident invasive breast carcinomas were identified through 2006. Multivariable Cox regression hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated for breast cancer risk factors by age (50-59, 60-69, ≥70 years).
Results: The only factor showing significant statistical heterogeneity by age (p(het) = 0.001) was menopausal hormone therapy duration, but trends were apparent across all ages and the strongest association prevailed among women 60-69 years. Although other risk factors did not show statistically significant heterogeneity by age, we did observe attenuated relations for parity and late age at first birth among older women [e.g., HR for age at first birth ≥30 vs. 20-24 = 1.62 (95% CI 1.23-2.14) for women 50-59 years vs. 1.12 (0.96-1.31) for ≥70 years]. In contrast, risk estimates associated with alcohol consumption and BMI tended to be slightly stronger among the oldest subjects [e.g., HR for BMI ≥35 vs. 18.5-24.9 = 1.24 (95% CI 0.97-1.58) for 50-59 years vs. 1.46 (1.26-1.70) for ≥70 years]. These differences were somewhat more pronounced for estrogen receptor positive and ductal cancers, tumors predominating among older women. Breast cancer family history, physical activity, and previous breast biopsies did not show divergent associations by age.
Conclusion: Although breast cancer risk factor differences among older women were not large, they may merit further consideration with respect to individualized risk prediction.